The present invention relates to an electric melting furnace for glassifying high-radioactive waste having been produced in a reprocessing plant.
The high-radioactive waste having been produced in the reprocessing plant is normally liquid and has an extremely high radioactivity. It is for this reason that for safely isolating highly radioactive waste from the human environment over a long period, the processing technic had to be developed wherein the waste and glass-making materials are melted into glass at high temperature, and poured into a canister as a container for solidification.
This means that the high-radioactive waste undergoes a suitable pre-treatment, and is first fed to an electric melting furnace (hereinafter referred to as melter) normally in liquid state together with glass material. Thus, in the melter, the high-radioactive waste becomes a high-temperature molten-glass together with the glass material. This molten-glass is put into a metal container called canister in a continuous or intermittent fashion. When filled with glass, the canister is sealed up, stored temporarily in storage facilities, and is scheduled to be finally buried into a deep geological formation for permanent disposal.
The high-radioactive waste and the glass material which are fed continuously to the melter (hereinafter referred to simply as material including both of them) are put in a state of covering the surface of the molten-glass in a melting tank made of bricks, and the evaporation of water in the waste, calcining glassifying reaction take place continuously by the transfer of heat from the molten-glass, and they are mixed with the existing molten-glass, becoming a homogeneous glass. Energy required to keep the molten-glass at high temperature is supplied by carrying a current across at least a pair of electrodes facing each other disposed in the molten-glass to Joule-heat the molten-glass existing inbetween.
For the protection of the operator from exposure, the melter is installed in a space called cell, and the operation, maintenance and exchange thereof are carried out by the remote manipulation. For this reason, the melter is so designed as to be smaller in size and light in weight whenever possible, and in the conventional melter for developing the technique of glassifying the high-radioactive waste, the inner capacity of the melting tank is so designed as to be also smaller whenever possible. This means that the depth of the melting tank is set at a minimum capable of disposing the above-described electrodes, and the bottom surface of the melting tank is designed to be nearly horizontal so that the inner capacity of the melting tank becomes small.
The high-radioactive waste contains the platinum-group elements such as Ru, Pd and Rh. These elements are hard to melt into glass and have large specific gravities, and therefore precipitate and accumulate on the bottom of the melting tank. Among these platinum-group elements, Pd and Rh are reduced and exist as metals in the glass, and Ru exists as metals or RuO.sub.2 crystal.
RuO.sub.2 is an oxide, but is a good electrical conductor, and reduced metals of Ru, Pd and Rh are, needless to say, good electrical conductors, and when such substances accumulate on the bottom of the furnace at high concentrations, the high-temperature inherent resistance value of the glass in the vicinity of the furnace bottom becomes smaller in comparison with that of the glass at the upper part (hereinafter the glass in the vicinity of the furnace bottom containing the platinum-group elements at high concentrations is referred to hereafter as furnace-bottom accumulation).
When the platinum-group elements accumulate on the bottom of the furnace, forming a good electrical conductor, in the conventional melter designed to have a shallow melting tank, a current flowing across the electrodes concentrates on the bottom of the furnace, and the temperature of the bottom of the furnace rises extraordinarily, and in reverse, the temperature of the glass at the surface of the melting tank falls and the ability of melting material is reduced. Also, since the surface of the bottom of the melting tank is nearly horizontal, the platinum-group elements accumulating on the bottom of the furnace do not flow even when the glass is made to flow down into the canister, and increasingly accumulate on the bottom of the furnace, and eventually no operation can be continued.
In order to stably glassify the high-radioactive waste containing the platinum-group elements in the melter of Joule-heating system, it is considered that the melter is required to provide the following two functions.
a. The platinum-group elements are hard to melt into glass, and the specific gravities thereof are about 10, while the specific gravities of the normal liquids of molten-glass is about 2.5 or more, and therefore they precipitate quickly in the liquid molten-glass, accumulating on the bottom of the furnace. The glass containing the platinum-group elements at high concentrations, that is, the furnace-bottom accumulation has a high-temperature inherent electric resistance value lower than that of the glass at the upper part thereof, and therefore when glass-melting of the high-radioactive waste is started, a good electrical conductor layer is formed on the bottom of the furnace in a short time.
Accordingly, the melter for glassifying the high-radioactive waste is required to be able to continue operation without trouble even when a good electrical conductor layer exists to some extent on the bottom of the furnace, that is, it is required to have a structure wherein the current flowing across electrodes does not concentrate selectively on the bottom of the furnace.
b. In the preceding paragraph, description is made on the necessity of a melter design having an electrodes arrangement capable of continuing operation even if a good conductive layer exists to some extent on the bottom of the furnace, but when the glass is poured into the canister, if only the furnace-bottom accumulation remains and continues to accumulate in the melting tank, naturally heating by means of carrying a current across electrodes is hindered.
Accordingly, a melter design is required wherein the bottom of the furnace has an inclination enough to remove the furnace-bottom accumulation by discharging it regularly or irregularly through an outlet.